Implement "lazy loading" of the directory tree

[Nicklas Nordborg]

On a server with lots of user's the file manager is almost impossible to use since the generation of the directory tree takes too long time (if it works at all). The directory tree should be lazily loaded, with only the top-level items shown to begin with. Subdirectories are loaded on demand as the user click on them.

This ticket depends on #1245.

[Nicklas Nordborg]

(In [4794]) References #1241: Implement "lazy loading" of the directory tree

I think it should be working now. Might need to check for additional cases were pages are linked into the middle of the tree.

[Nicklas Nordborg]

(In [4795]) References #1245: Add support for lazy loading of nodes in JoustTree?

Removed debug test case. Seems to work good for the directory tree (see #1241).

[Nicklas Nordborg]

(In [4796]) References #1241: Implement "lazy loading" of the directory tree

We can display the path even on directories where the current user is lacking read permission.

[Nicklas Nordborg]

(In [4798]) Fixes #1241: Implement "lazy loading" of the directory tree

Forgot this file in the first checkin.